HIV invades the brain shortly after infection. HIV invasion of the brain is accompanied by the infiltration of CD8+ T cells. However, the role of CD8+ T cells, whether pathogenic or protective, in the CNS is not clear. Our lab has previously identified a unique subtype of CD8+ T cells which upregulate CD4 on the cell surface after activation (termed DP T cells). These cells have also been identified in HIV+ patients, and are enriched in Long Term Non-Progresssor HIV+ Patients where they account for 60% of the HIV+ CD8+ Tetramer+ response. CD4 expression on the surface of CD8+ T cells is dependent on enhanced expression of ?-catenin. Our central hypothesis is that DP cells in the CNS will have a paradoxical role. On one hand, DP cells will be more activated/inflammatory than their single positive CD8 counterparts, and as such will be more effective in controlling HIV infection. However, the highly activated status of DP cells in the CNS could lead to acute bystander damage of resident brain cells. We will utilize both in vivo and in vitro methods to determine the role of activated CD8 single positive and DP T cells in the brain. Co-culture experiments of neurons and/or astrocytes with activated CD8 Single Positive T cells or DP T cells will tell us the effect of this highly activated population on brain resident cell function and survival. We have previously established that supernatant from activated CD8 single positive T cells, enriched in DP T cells, results in activation of astrocytes as shown by upregulation of HLA-DR expression and IFN? production by astrocytes. We will also utilize the NOD/SCID/IL-2rc?-/- Human Peripheral Blood Lymphocyte (NSG-huPBL) reconstituted mouse model of HIV infection to determine how frequency of CD8+ and DP T cells correlate with HIV burden in the brain. We have shown that DP T cells and CD8+ T cells can be identified in the brain of NSG-huPBL mice. Additionally, HIV infected CD4 single positive cells and DP T cells were identified in the brain and spleen. CD4 single positive and DP T cells were both susceptible to HIV infection but only CD4 single positive T cells were depleted. Although DP T cells in the CNS are a smaller population than CD8 SP T cells, we have previously shown that they make up approximately 60% of HIV-specific responses, indicating that despite their lower frequency in comparison to CD8 SP T cells they are highly enriched in anti-HIV responses and are likely to be highly activated in the CNS, driving anti-HIV responses but also leading to bystander damage to astrocytes and neurons. Collectively, our studies will determine the impact of CD8+ T cells, especially a unique subset of CD8+ T cells (DP), on resident brain cells and establish the mechanism by which these highly activated T cells affect the CNS in the context of HIV infection. Findings from these studies will be informative regarding basic biology of these cells as well as guide novel therapeutic strategies exploiting their role in CNS.